Logarithmic Transformations!

Okay so I'm supposed to perform an experiment where we take 20 coins and toss them removing heads in each and counting the remaining coins, till all the coins have been dropped. The values i got were:
TOSS COINS
0 : 20
1 : 9
2 : 4
3 : 3
4 : 0
(a) Plot these values on a graph. (I believe toss values wud be on x-axis and coins on y-axis as it's the dependent variable.)
(b) State an equation that best fits the results (use n to represent the number of the toss)
[Here I think it's some sort of decay equation like y= x(1/2)^n But using this, im not getting the required answers :S ]
(c) What physical phenomenon does this model represent?
(d) State the exponential equation that would predict the number of remaining coins if you began with N° coins ?